1. Field of the Invention
The present invention relates to a zoom lens system, and more particularly, to a small-sized zoom lens system for use in a lens shutter camera and a video camera.
2. Description of the Prior Art
In most of the known three-unit zoom lens systems of positive, positive, negative configuration, a first lens unit includes at least two lens elements, a second lens unit includes at least four lens elements, and a third lens unit includes two or three lens elements.
On the other hand, a three-unit zoom lens system of positive, positive, negative configuration has been proposed in which aberrations are corrected through the use of aspherical surfaces in the optical system to thereby reduce the number of lens elements. For example, U.S. Pat. No. 5,087,988 discloses a zoom lens system in which each lens unit includes one lens element. U.S. Pat. No. 5,325,235 discloses a zoom lens system having a total of four lens elements in which a first lens unit includes one lens element, a second lens unit includes two lens elements and a third lens unit includes one lens element. Japanese Laid-open Patent Application No. H5-188296 discloses a zoom lens system having a total of four or five lens elements in which a second lens unit includes one or two lens elements and a third lens unit includes one lens element. U.S. Pat. No. 5,434,870 discloses a zoom lens system having a total of four lens elements in which a first lens unit includes two lens elements, a second lens unit includes one lens element and a third lens unit includes one lens element. U.S. Pat. No. 5,424,870 discloses a zoom lens system having a total of six lens elements in which each lens unit has two lens elements.
U.S. Pat. No. 5,196,962 discloses a three-unit zoom lens system of positive, positive, negative configuration having a total of six lens elements in which a second lens unit is formed of two lens elements through the use of a gradient index lens (GRIN lens) in the second lens unit.
The GRIN lens is an optical device which has long attracted attention as its optical characteristic (i.e. a characteristic that light advances while being bend in the lens medium) and the possibility of dispersion control are considered highly effective in reducing the number of lens elements constituting the optical system and in reducing the size of the lens system. In particular, since the control of chromatic aberration and Petzval sum which cannot be performed through the aspherical surface can be performed through the GRIN lens, the GRIN lens is expected to have more advantages than the aspherical surface.
In the conventional zoom lens systems having aspherical surfaces, it is impossible to reduce the total number of lens elements of the optical system while reducing the size of the optical system and increasing the magnification. For example, if the first lens unit (i.e. the most object side lens unit) is formed of one lens element, chromatic aberration and Petzval sum generated in the lens unit cannot be sufficiently corrected only through homogeneous lenses, so that a smaller size and a higher magnification cannot be achieved (particularly, the realization of a high magnification is impossible). In addition, in the conventional three-unit zoom lens systems of positive, positive, negative configuration, since the number of lens elements of the second lens unit should be increased to correct aberrations generated in the lens unit, it is extremely difficult to reduce the total number of lens elements of the optical system. This will hereinafter be described in further detail.
In the zoom lens systems disclosed in U.S. Pat. Nos. 5,087,988 and 5,325,235, and Japanese Laid-open Patent Application No. H5-188296, since only homogeneous lenses are used, aberrations (particularly, chromatic aberration and Petzval sum) are not sufficiently corrected, so that only small zoom ratios and large F numbers are achieved. In the zoom lens system disclosed in U.S. Pat. No. 5,434,711, a magnification as high as 2.6.times. is achieved with a smaller number of lens elements through the use of aspherical surfaces. However, it can hardly be regarded as having high performance.
In the zoom lens system disclosed in U.S. Pat. No. 5,424,870, the aspherical surfaces are incapable of correcting chromatic aberration and Petzval sum, so that each lens unit should include at least two lens elements. This is because in zoom lens systems, in order to correct aberrations in the entire zoom range, it is necessary that at least chromatic aberration and Petzval sum should be corrected in each of the lens units constituting the zoom lens system.
In the zoom lens system disclosed in U.S. Pat. No. 5,196,962, the number of lens elements is reduced through the use of a GRIN lens in the second lens unit. However, since the GRIN lens comprises only spherical lenses, the degree of freedom of the aberration correction is insufficient, so that the number of lens elements of the second lens unit is reduced only to two.
When a fixed focal length lens is constituted by a spherical GRIN lens, it is considered impossible to form it of one lens element since the degree of freedom for the third-order aberration correction is insufficient (see L. G. Atkinson et al., "Design of a gradient-index photographic objective", Appl. Opt. Vol. 21, 1982, pp 993-998). This applies to zoom lens systems. That is, if the lens units constituting a zoom lens system are each formed of one lens element, it is impossible to correct aberration because of the insufficient degree of freedom of the third-order aberration correction, so that it is impossible to form each of the lens units of the zoom lens system of one lens element.